1. Field of the Invention
This invention relates to a chaff molding prepared by molding a chaff and a polyvinyl alcohol-based resin (hereinafter, polyvinyl alcohol is referred to as PVA). For more details, the invention relates to a chaff molding excellent in a compressive strength and a sound absorption in an audible range.
2. Description of the Related Art
In recent years, attempts have been energetically made to use an agricultural by-product as a biomass resource. Particularly, since the generating quantity of the by-product of rice as the main crops in Japan, such as a rice straw, a chaff, a rice bran and the like, is vast, their effective application is an important challenge. More particularly, the chaff is useful as an industrial material because it contains a large amount of silica, and for example, an examination has been carried out on the application of a chaff pulverization product obtained by pulverizing this as a filler for reinforcing a synthetic resin. However, since the chaff has a tough shell tissue consisting of cellulose, a pretreatment is required for efficiently pulverizing this. For example, a method in which the chaff is treated with enzyme and then mechanically pulverized (e.g., see Patent Reference 1: JP-A-2005-199133) and the like have been proposed.
As an application of the thus obtained chaff pulverization product, this Patent Reference 1 proposes a molding prepared using, as a binder, a PVA-based resin which is biodegradable similar to the chaff and has excellent affinity for cellulose and silica in the chaff. In addition, as a specific production method, its Examples show a method in which a mixture of a chaff pulverization product with a PVA-based resin water solution is cooled to 0° C. or lower to effect gelation and then dried, and describe that the thus obtained molding has a strength which is sufficient as a structural material.
In such a gel, the continued vinyl alcohol structural units in the PVA molecule chain form a microcrystal in the molecular and between the molecular, the crystal grows during the rising temperature process, and the three-dimensional cross-linked form of PVA obtained using such a crystal structure as a junction point contains water. Thus, in order to obtain a hard gel having a high cross-linking density, it is desirable to use a fully saponified PVA-based resin having large content of vinyl alcohol structural unit.
However, because of its high crystallinity, the fully saponified PVA-based resin requires its dissolution at a high temperature for a prolonged period at the time of preparing its water solution, and poses a tendency in that the PVA-based resin crystallizes with time in the thus obtained water solution and the water solution becomes viscous and undergoes gelation. In addition, there is a case in which a micro junction remained due to imperfect dissolution and a heterogeneously gelled part caused by the crystal formed during preservation become a structural defect in the molding and thereby become a cause of reducing a mechanical strength and the like.
Accordingly, when the gelation phenomenon by cooling of the PVA-based resin water solution is used in the production of the molding, a proper degree of saponification of the PVA-based resin to be used is about 99% by mol, and a PVA-based resin having the degree of saponification of 99% by mol is used in Example 1 of JP-A-2005-199133. When a PVA-based resin having a higher degree of saponification than this is used, the aforementioned heterogeneous gelation during the production process is apt to occur, and when the degree of saponification is low on the contrary, it becomes difficult to obtain a proper gel.
In addition, even when the PVA-based resin having the degree of saponification of about 99% by mol is used, the gel is not formed by one cooling, and even when the gel is obtained, cross-link formation is not sufficient so that the gel is dissolved or the shape is lost in some case in the subsequent heat drying step. Thus, in order to form a hard three-dimensionally cross-linked gel, it was necessary in general to form the hard gel by increasing amount of micro junctions and increasing junction points, by repeating a cooling-rising temperature cycle in which once cooling is performed to 0° C. or lower to effect gelation, and then rising temperature is performed to 0 to 5° C. and cooling is performed again. However, two or more times of repetition of such a cooling-rising temperature cycle has a problem in terms of consuming energy and time.
For the purpose of solving such problems, a method for effecting gelation by cooling under a compressed state has been proposed (e.g., see Non-patent Reference 1: Report on research results, Development of an environment-adaptation type “chaff/PVA composite material” making use of chaff pulverization techniques, H17, Industry-academic-government new technology development project, proposed and publicly offered by Shiga prefecture, School of Engineering, The University of Shiga Prefecture, page 47 to 78). When such a compression cooling method is used, a hard gel can be obtained by one cooling in the case of the use of a PVA-based resin having a degree of saponification of about 99% by mol.